The preparation of aldehydes and alcohols by the addition of carbon monoxide and hydrogen to olefinic double bonds (hydroformylation) is known. The reaction is catalyzed by metals of the 8th subgroup of the Periodic Table or their compounds which form carbonyls or hydridocarbonyls under the reaction conditions. While formerly cobalt and cobalt compounds were used almost exclusively as catalysts, rhodium catalysts are increasingly favored today even though rhodium is many times more expensive than cobalt. Rhodium is used alone or in combination with complexing agents, e.g. organic phosphines. While the oxo synthesis requires reaction pressures of 25 to 30 MPa with rhodium as a catalyst, pressures of 1 to 5 MPa are sufficient if rhodium complex compounds are used.
There are clear advantages to the use of rhodium catalysts in many cases. They have higher activity and selectivity, and also permit uncomplicated operation of the production facility, in particular with regard to the performance of the synthesis and the removal of the products from the reactor. Finally, the classic oxo process based on cobalt catalysts can be converted to rhodium catalysts using the existing apparatus, in many cases with little additional capital expenditure.
However, the loss-free or at least nearly loss-free separation and recovery of rhodium causes considerable problems, regardless of whether it is used as a catalyst with or without an additional complexing agent. After completion of the reaction, the rhodium is to be found dissolved in the hydroformylation product as a carbonyl compound which can also contain further ligands.
For work-up, the oxo raw product is normally pressure-relieved in several stages by reducing the synthesis pressure (which is approximately 1 to 30 MPa depending on the type of rhodium catalyst used) initially to about 0.5 to 2.5 MPa and the synthesis gas dissolved in the raw product is then released. Thereafter, the pressure can be reduced to normal. The rhodium is separated either directly out of the raw product or out of the residue of the raw product distillation. The first route is taken when rhodium has been used as a catalyst in the previous hydroformylation stage without an additional complexing agent.
The second variation is used when the rhodium catalyst contains not only carbon monoxide but also other ligands, e.g. phosphines or phosphites in complex bonding. The latter can also be used when hydroformylation is carried out with rhodium alone but a complexing agent is added to the raw product to stabilize the rhodium after the pressure has been relieved. It must always be remembered that the noble metal is only present in the raw product in a concentration of a few ppm, therefore it must be separated very carefully. Additional difficulties can be caused by the rhodium partly changing into metallic form or forming polynuclear carbonyls during pressure relief, especially when it is used without a ligand. Then a heterogeneous system is formed which consists of the liquid organic phase and the solid phase containing rhodium or rhodium compounds.
Several processes for separating rhodium from the oxo raw product are known. According to the procedure described in DE 33 47 406 A1, rhodium is recovered from the oxo raw product by extraction with complexing reagents. According to a preferred embodiment, sulfonates and carboxylates of organic phosphines are used as complexing agents.
Sulfonated triphenylphosphines, e.g. salts of triphenylphosphine trisulfonic acid, are particularly suitable complexing reagents. The complex compounds formed from rhodium and the sulfonates or carboxylates of the organic phosphines are water-soluble. Thus, the rhodium can be extracted from the oxo raw product, i.e. the organic phase, with an aqueous solution of the substituted phosphine. The aqueous, rhodium-containing phase can be separated from the organic product mixture by simple decantation. High rhodium concentrations can be achieved in the solution of the complexing agent by recirculation.
In order to accelerate and complete extraction of the rhodium from the organic phase and its transfer to the aqueous phase, a solubilizer is added to the aqueous solution of the complexing agent according to DE 34 11 034 A1. This is particularly effective because it changes the physical properties of the interface between the two liquid phases. Thus, the introduction of the aqueous extracting agent into the product phase and the transfer of the rhodium from the product phase into the aqueous complexing agent phase is accelerated, extraction is simplified, and the apparatus required is reduced.
The higher the concentration of the solubilizer in the aqueous phase, the more rhodium is extracted. However, the amount of solubilizer cannot be increased infinitely because it affects the aqueous solution of the extracting agent and impairs its stability. Therefore, the process described in DE 34 43 474 A1 uses quaternary ammonium salts of sulfonated triphenylphosphines as complexing agents. However, it is very costly to produce these salts, which makes the extraction process not always economically feasible.
Various processes are also known for separating the rhodium from the distillation residues of the oxo raw product. According to the process in EP 15 379 A1, the rhodium-ligand catalysts used in the oxo synthesis are oxidized, e.g. with air in the presence of aldehyde, and the solid reaction products thus formed are removed. The solution obtained can be reused as a catalyst after the liquid has been replenished. This procedure has its limitations because the residues formed during distillation are not separated.
The subject of U.S. Pat. No. 4,400,547 is a hydroformylation process in the presence of unmodified rhodium as a catalyst. After a phosphorus ligand such as triphenylphosphine has been added to the oxo raw product, the aldehyde is distilled off. Then the distillation residue is treated with oxygen to split off the ligand from the complex compound again and to recover the rhodium in its active form. Therefore, complete separation of the rhodium and distillation residue is not possible with this procedure.
Separation of noble metals such as rhodium from high-boiling hydroformylation residues is described in US-PS 3 547 964. In this process the residues are treated with hydrogen peroxide in the presence of acids such as formic acid, nitric acid or sulfuric acid. Owing to the high price of hydrogen peroxide and the problems in handling it, the commercial applications of this process are limited.
According to DE 24 48 005 C2, a distillation residue containing rhodium is initially treated with acids and peroxides. Then excess peroxides are destroyed by heating and the aqueous solution containing the catalyst metal is reacted, in the presence of a water-soluble organic solvent, with hydrohalic acid or alkali halides, as well as with tertiary phosphines and carbon monoxide or compounds which split off carbon monoxide. This procedure again requires the use of peroxides accompanied by the above-mentioned economic disadvantages and the use of halogen-resistant materials.
Therefore, the problem was to a process which permits the recovery of rhodium from residues obtained during the distillation of reaction mixtures from the oxo synthesis as simply and completely as possible.